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Organic Chemistry

Organic Chemistry. William H. Brown & Christopher S. Foote. Aromatics II. Chapter 20. Chapter 21. Reactions of Benzene. The most characteristic reaction of aromatic compounds is substitution at a ring carbon. Reactions of Benzene. Electrophilic Aromatic Sub.

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Organic Chemistry

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  1. Organic Chemistry William H. Brown & Christopher S. Foote

  2. Aromatics II Chapter 20 Chapter 21

  3. Reactions of Benzene • The most characteristic reaction of aromatic compounds is substitution at a ring carbon

  4. Reactions of Benzene

  5. Electrophilic Aromatic Sub • Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile • We study • several common types of electrophiles • how each is generated • the mechanism by which each replaces hydrogen

  6. Chlorination Step 1: formation of a chloronium ion

  7. Chlorination Step 2: attack of the chloronium ion on the aromatic ring Step 3: proton transfer regenerates the aromatic character of the ring

  8. EAS: General Mechanism • A general mechanism • General question: what is the electrophile and how is it generated?

  9. Nitration • The electrophile is NO2+, generated in this way

  10. Nitration Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile) Step 2: proton transfer regenerates the aromatic ring

  11. Nitration • A particular value of nitration is that the nitro group can be reduced to a 1° amino group

  12. Sulfonation • Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide

  13. Friedel-Crafts Alkylation • Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group

  14. Friedel-Crafts Alkylation Step 1: formation of an alkyl cation as an ion pair Step 2: attack of the alkyl cation on the aromatic ring Step 3: proton transfer regenerates the aromatic ring

  15. Friedel-Crafts Alkylation • There are two major limitations on Friedel-Crafts alkylations 1. carbocation rearrangements are common

  16. Friedel-Crafts Alkylation • the isobutyl chloride/AlCl3 complex rearranges to the tert-butyl cation/AlCl4- ion pair, which is the electrophile

  17. Friedel-Crafts Alkylation 2. F-C alkylation fails on benzene rings bearing one or more of these strongly electron-withdrawing groups

  18. Friedel-Crafts Acylation • Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group

  19. Friedel-Crafts Acylation • The electrophile is an acylium ion

  20. Friedel-Crafts Acylation • an acylium ion is a resonance hybrid of two major contributing structures • F-C acylations are free of a major limitation of F-C alkylations; acylium ions do not rearrange

  21. Friedel-Crafts Acylation • A special value of F-C acylations is preparation of unrearranged alkylbenzenes

  22. Other Aromatic Alkylations • Carbocations are generated by • treatment of an alkene with a protic acid, most commonly H2SO4, H3PO4, or HF/BF3

  23. Other Aromatic Alkylations • by treating an alkene with a Lewis acid • and by treating an alcohol with H2SO4 or H3PO4

  24. Di- and Polysubstitution • Existing groups on a benzene ring influence further substitution in both orientation and rate • Orientation: • certain substituents direct preferentially to ortho & para positions; others direct preferentially to meta positions • substituents are classified as either ortho-para directingor meta directing

  25. Di- and Polysubstitution • Rate • certain substituents cause the rate of a second substitution to be greater than that for benzene itself; others cause the rate to be lower • substituents are classified as activating or deactivating toward further substitution

  26. Di- and Polysubstitution • -OCH3 is ortho-para directing

  27. Di- and Polysubstitution • -NO2 is meta directing

  28. Di- and Polysubstitution

  29. Di- and Polysubstitution • From the information in Table 21.1, we can make these generalizations • alkyl, phenyl, and all other groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other groups are meta directing • all ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating

  30. Di- and Polysubstitution • the order of steps is important

  31. Theory of Directing Effects • The rate of EAS is limited by the slowest step in the reaction • For almost every EAS, the rate-determining step is attack of E+ on the aromatic ring to give a resonance-stabilized cation intermediate • The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction

  32. Theory of Directing Effects • For ortho-para directors, ortho-para attack forms a more stable cation than meta attack • ortho-para products are formed faster than meta products • For meta directors, meta attack forms a more stable cation than ortho-para attack • meta products are formed faster than ortho-para products

  33. Theory of Directing Effects • -OCH3; assume meta attack

  34. Theory of Directing Effects • -OCH3: assume ortho-para attack

  35. Theory of Directing Effects • -NO2; assume meta attack

  36. Theory of Directing Effects • -NO2: assume ortho-para attack

  37. Activating-Deactivating • Any resonance effect, such as that of -NH2, -OH, and -OR, that delocalizes the positive charge on the cation intermediate lowers the activation energy for its formation, and has an activating effect toward further EAS • Any resonance or inductive effect, such as that of -NO2, -CN, -CO, or -SO3H, that decreases electron density on the ring deactivates the ring toward further EAS

  38. Activating-Deactivating • Any inductive effect, such as that of -CH3 or other alkyl group, that releases electron density toward the ring activates the ring toward further EAS • Any inductive effect, such as that of -halogen, -NR3+, -CCl3, or -CF3, that decreases electron density on the ring deactivates the ring toward further EAS

  39. Halogens • for the halogens, the inductive and resonance effects run counter to each other, but the former is somewhat stronger • the net effect is that halogens are deactivating but ortho-para directing

  40. Nucleophilic Aromatic Sub. • Aryl halides do not undergo nucleophilic substitution by either SN1 or SN2 pathways • They do undergo nucleophilic substitutions, but by mechanisms quite different from those of nucleophilic aliphatic substitution • Nucleophilic aromatic substitutions are far less common than electrophilic aromatic substitutions

  41. Benzyne Intermediates • When heated under pressure with aqueous NaOH, chlorobenzene is converted to sodium phenoxide. Neutralization with HCl gives phenol.

  42. Benzyne Intermediates • the same reaction with 2-chlorotoluene gives a mixture of ortho- and meta-cresol • the same type of reaction can be brought about using of sodium amide in liquid ammonia

  43. Benzyne Intermediates • -elimination of HX gives a benzyne intermediate, that then adds the nucleophile to give products

  44. Nu Addition-Elimination • when an aryl halide contains electron-withdrawing NO2 groups ortho and/or para to X, nucleophilic aromatic substitution takes place readily • neutralization with HCl gives the phenol

  45. Meisenheimer Complex • reaction involves a Meisenheimer complex intermediate

  46. Prob 21.7 Write a mechanism for each reaction.

  47. Prob 21.8 Offer an explanation for the preferential nitration of pyridine in the 3 position rather than the 2 position.

  48. Prob 21.9 Offer an explanation for the preferential nitration of pyrrole in the 2 position rather than in the 3 position.

  49. Prob 21.15 Predict the major product(s) from treatment of each compound with HNO3/H2SO4.

  50. Prob 21.16 Account for the fact that N-phenylacetamide is less reactive toward electrophilic aromatic substitution than aniline.

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